1. Field of the Invention
This invention relates to improvements in an apparatus and method for controlling an electric car driven by a linear induction motor.
2. Description of the Prior Art
Attention is now directed to a linear-motor electric car in which a primary coil forming a primary side of a linear induction motor is mounted on the electric car, and a reaction plate forming a secondary side of the linear induction motor is laid on the ground. The linear-motor electric car is supported by wheels to which no powering torque is transmitted, and a force propelling the linear-motor electric car is provided by a powering torque produced by the linear induction motor. On the other hand, a force decelerating the linear-motor electric car is provided by the combination of an electrical braking torque produced by the linear induction motor and a mechanical braking torque imparted to the wheels from mechanical brakes. (Pneumatic brakes are commonly used as the mechanical brakes.)
The greatest advantage of this linear-motor electric car is that the dimension between the floor of the electric car and the ground can be made small. This is especially advantageous in that the cross-sectional area of a subway tunnel can be markedly decreased, thereby contributing to realization of a subway car having a very small cross-sectional shape.
The linear induction motor driving the electric car has such an inherent problem that the length of the air gap between its primary and secondary sides varies greatly when the electric car is running. Such a variation of the air gap length impairs the constant torque characteristic demanded for the electric car. With a view to solve such a problem, a method is proposed in JP-A-No. 61-199404. According to the disclosure of the cited publication, a variation of the air gap length between the primary and secondary sides of a linear induction motor is detected so as to correct the slip frequency and motor current of the linear induction motor on the basis of the detected variation of the air gap length.
However, the prior art method for correcting the slip frequency and current of the linear induction motor on the basis of the detected variation of the air gap length has a first and a second problem as described below.
In the first place, it is extremely difficult to accurately detect the variation of the air gap length varying due to many different factors. The following are factors considered to cause variations of the air gap length:
(1) Wear of wheels
(2) Wear of rails
(3) Laying error of reaction plate relative to rails
(4) Distorsion of rails
(5) Deflection of reaction plate due to, for example, attractive force of linear induction motor
(6) Fall of wheels into joint gap between rails
(7) Vibration of body of electric car during running
(8) Others
In order to detect all of the causes of variations of the air gap length attributable to these factors, detection of, for example, vertical movement of the truck relative to the rails is not sufficient, and a gap sensor must be mounted on the primary coil provided on the electric car so as to directly detect a variation of the air gap length between the primary coil and the reaction plate laid on the ground. However, the gap sensor presently available cannot be disposed adjacent to the primary coil generating a strong magnetic field and must be disposed at a position considerably distant from the primary coil. Therefore, the above demand cannot be satisfied.
In the second place, in a linear-motor electric car, power is usually supplied from a single power converter to a plurality of linear induction motors, for example, four or eight linear induction motors. Further, each of these linear induction motors has a considerable length in the advancing direction of the electric car. Therefore, unless a variation of the air gap length in each of the linear induction motors is accurately detected, the combined torque of the linear induction motors driving the electric car cannot be accurately controlled. Thus, a plurality of air gap sensors are inevitably required, and, from this aspect too, the prior art method is insufficient in the accuracy and reliability of torque control.
Therefore, the prior art method using the air gap sensor is difficult to compensate torque variations in a linear-motor electric car and has not been put into practical use.